(1) Field of the Invention
This invention relates to methods and apparatus for supplying a treating solution such as photoresist or developer to substrates such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or glass substrates for optical disks (hereinafter referred to simply as substrates or as wafers). More particularly, the invention relates to a technique of treating the surface of each substrate by supplying a treating solution in a predetermined fixed quantity thereto based on a prestored processing program with a plurality of instructions including a supply start instruction and a supply stop instruction for performing a series of processes.
(2) Description of the Related Art
A conventional treating solution supplying method of the type noted above will be described with reference to a time chart shown FIG. 1.
This time chart corresponds to a processing program (also called a spin coat program or recipe) prepared beforehand in accordance with a desired film thickness and the like. At a point of time T.sub.S at which a substrate remains still, for example, a supply start instruction is executed to start supplying a photoresist solution as a treating solution. At a point of time T.sub.E when a predetermined supplying period T.sub.SU elapses, a supply stop instruction is executed to stop supplying the photoresist solution. The above method in which the photoresist supply is started and stopped while the substrate is maintained still is hereinafter called the "static method".
At a point of time ti which is a predetermined period after stopping the photoresist supply to the substrate as noted above, the substrate is spun with an acceleration to a first rotational frequency R1 (e.g. 900 rpm). The spin at the first rotational frequency R1 is maintained for a predetermined period whereby the photoresist solution is spread substantially over the entire surface of the substrate. Next, the substrate is spun at a second rotational frequency R2 (e.g. 3,000 rpm) higher than the first rotational frequency R1 for a predetermined period. Consequently, the photoresist solution is spread completely over the entire surface of the substrate, and a superfluous part of the photoresist solution is dispelled, thereby forming a photoresist film of desired thickness uniformly over the entire surface of the substrate.
A mechanism for supplying the photoresist solution, e.g. a pump for feeding the photoresist solution to a nozzle, usually includes a filter disposed at an output side thereof. This filter is provided primarily for the purpose of protecting substrates from contamination when impurities are contained in the photoresist solution. However, an increasing pressure loss occurs across the filter as its mesh becomes clogged through use, for example. Such an increasing pressure loss results in a reduced flow velocity of the photoresist solution supplied. Then, even if the pump is driven for the predetermined supplying period T.sub.SU to supply the photoresist solution as noted above, the photoresist solution is supplied during the supplying period T.sub.SU in a quantity less than what is intended. Inconveniences are encountered where, as days go by, substrate treatment based on the processing program fails to form films of uniform thickness or of desired thickness.
There has been an ever-intensifying movement recently, with a view to a reduction in the cost of manufacturing semiconductor devices, environmental protection and so on, to limit to a necessity minimum the supply of photoresist solution which was formerly was supplied in a lavish way, thereby to minimize the quantity of photoresist solution dispelled and discarded (the movement being called resist saving also). Such resist saving could give rise to the worst situation where the surface of a substrate is not fully covered by the photoresist solution supplied in a reduced quantity.
The following method has been proposed to solve the problem experienced with the above method based on supplying the photoresist solution at a fixed flow velocity.
That is, the quantity of photoresist solution fed by the pump is controlled to supply the photoresist solution constantly in a predetermined quantity. By controlling the quantity of supply in place of the control based on the photoresist supplying period, the photoresist solution may be supplied in the predetermined quantity to the substrate despite an increased pressure loss. Thus, the photoresist solution may be supplied in a predetermined fixed quantity over a long period of time, to avoid the inconvenience due to energy saving noted above.
However, this conventional method has the following drawback.
The photoresist supply is started at the point of time T.sub.S for executing the supply start instruction, and stopped at the point of time T.sub.E for executing the supply stop instruction when the photoresist solution has been supplied in a predetermined fixed quantity. The supplying period T.sub.SU between these instructions becomes extended due to pressure loss increases occurring with passage of some days. This results in shortening of the intended period from the point of time T.sub.E for stopping the photoresist supply on the supply stop instruction to the point of time ti for starting a substrate spin. In an extreme case, the supply period is extended to a period (T.sub.SU1) as indicated in a dotted line in FIG. 1, so that the photoresist supply is stopped after the substrate is started spinning. This is contrary to the intended processing method, i.e. the "static method", in which the photoresist supply should be completed while the substrate remains still prior to a spin.
The processing program is prepared through experiment using dummy substrates with the same surface condition as product substrates. The experiment is repeated by varying conditions such as timing of starting and stopping the photoresist supply, rotational frequencies R1 and R2, and the periods for maintaining the rotational frequencies R1 and R2. The program is based on optimal conditions found through the experiment to form films of desired thickness, and that uniformly over the entire surface of each substrate. Thus, in the event of any mistiming, the substrate is treated in a way deviating from the optimum conditions set by the program.
Methods of supplying the photoresist solution other than the above "static method" include a method in which the supply start instruction is executed while the substrate is spun at the first rotational frequency R1, and the supply stop instruction is executed to complete the supply when the photoresist solution has been supplied in a fixed quantity (which is hereinafter called the "dynamic method"), and a method in which the supply start instruction is executed while the substrate remains still, and the supply stop instruction is executed, when the photoresist solution has been supplied in a fixed quantity, to complete the supply after the substrate is spun with an acceleration to the first rotational frequency R1. The latter is a combination of the "static method" and "dynamic method", and will be called "stamic method" hereinfter.
In the "dynamic method" and "stamic method", an extended supplying period results in shortening of the period between the point of time for stopping the photoresist supply and the point of time for starting to accelerate the spin to the second rotational frequency R2. Thus, as in the "static method", the substrate is treated in a way deviating from the optimum conditions.